1. Field of the Invention
The invention relates to a process for preparing aluminum salts of saturated or unsaturated cyclic phosphinic acids, in particular aluminum salts of 1-hydroxydihydrophosphole oxides and of 1-hydroxyphospholane oxides.
2. Description of the Prior Art
Salts from phosphinic acids have been known per se for a relatively long time and are recommended in particular as flame-retardant additives for thermoplastics, such as polyesters or polyamides. For example, DE-A1-2252258 describes alkali metal salts of phosphonic acids. However, they have to be added in comparatively large amounts, and in some cases have an adverse corrosion-promoting effect on the processing machinery,
EP-A3-0699708 discloses polyester molding compositions which contain calcium phosphinates or aluminum phosphinates of the following formulae: ##STR1##
where
R.sup.1 and R.sup.2 are C.sub.1 -C.sub.6 -alkyl, linear or branched, or phenyl; PA0 R.sup.3 is C.sub.1 -C.sub.10 -alkylene, linear or branched, or arylene, or alkylarylene, or arylalkylene; PA0 M is a calcium or aluminum ion; PA0 m is 2 or 3; PA0 n is 1 or 3; PA0 x is 1 or 2
The aluminum salts described in this European patent application and derived from the phosphinic acids also mentioned therein, require for their preparation a comparatively long reaction time, specifically 24, and in some cases even 65, hours.
Although a very large number of compounds derived from phosphinic acids, and preparation processes for these compounds, are already known, there is still a need for improved processes for preparing suitable salts derived from phosphonic acids and capable of being used as agents for improving the flame retardancy of plastics.
It is therefore an object of the invention to provide a process which permits the preparation of useful salts derived from phosphinic acids and capable of being used as flame retardants. Another object of the invention is to provide a process which operates cost-effectively and gives high yields with short reaction times. A further object of the invention is to provide a process which operates in an environmentally sound manner and which gives rise to no, or only very small, amounts of substances which have to be removed and disposed of in isolating the end product.